A known design of wind turbine has a tower-mounted propeller-like rotor, typically comprising two or three blades that radiate perpendicular to the axis of rotation of the rotor, which is generally aligned with the prevailing wind direction. Other known designs of wind turbine have blades arranged in a frame or cage and the blades extend along the axis of rotation of the rotation, which is aligned transverse to the prevailing wind direction. Examples of the latter are Savonius rotors or Darrieus rotors, as described in U.S. Pat. No. 1,697,574 and U.S. Pat. No. 1,835,018 respectively, as well as a basic paddle-wheel type of rotor.
Wind turbines are typically designed to maximise their power output in relatively high wind speed conditions. However, under substantially laminar flow, aerodynamic lift of an aerofoil increases approximately quadratically with relative wind speed. Accordingly, and disadvantageously, such wind turbines are vulnerable to damage in extremely high winds, and produce low power output in low wind speeds.
Control of rotor rotational speed is of critical importance, and can be particularly challenging during extremely high wind speeds. Accordingly, wind turbines are typically provided with control mechanisms in the form of control electronics, braking systems, or shuttering or regulation elements to shield the rotor from the wind.
Disadvantageously, wind turbines are vulnerable to damage in the case of a failure or disconnection of their control electronics, which can result in their spinning out of control, dramatically increasing component wear and reducing the lifetime of the wind turbine.
Disadvantageously, during highly variable wind conditions high torques can be produced suddenly causing to accelerate more rapidly than their control electronics and/or braking systems can respond or to exceed the braking capacity available. This problem has most notably arisen in relation to large-scale wind turbines having propeller-like rotors.
There has been increasing interest in mounting wind turbines on the roof ridges of buildings, to take advantage of the wind focus effect, by which the wind passing up a roof to a ridge is compressed into a high speed Aeolian flow band. Empirical measurement has shown that for a usual 30 degree to 45 degree pitched roof, this band extends approximately 300 mm above the ridge top and 450 mm forward along the roof facing the incident wind. At this point, measured speeds may be of the order of three times that of the prevailing wind speed. However, the unpredictable aerodynamic influences of nearby buildings and trees can lead to variable wind conditions, complicating wind turbine design.
WO2011010159 discloses a roof ridge mountable wind turbine having an elongate cylindrical wind turbine rotor, a variable flow regulator within the rotor housing, and a control system, such that the regulator can be used to variably reduce the wind flow through the rotor. It is also known to provide shutters that can be closed in high wind conditions.
Disadvantageously, such flow regulator mechanisms increase the manufacturing complexity and cost of the wind turbines. Further, due to the mechanical demands that such mechanisms require to withstand, they typically require robust engineering that increases the weight of the wind turbine assembly, increasing installation complexity and cost, as well as restricting the number of roofs on which they can be installed due to limitations of the maximum load that each roof can bear.
Accordingly, there remains a need for a reliable, lower weight and low complexity wind turbine assembly that can withstand high wind conditions.